In aqueous ink indirect printing, an aqueous ink is jetted on to an intermediate imaging surface, which can be in the form of a blanket. The ink is partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. To ensure excellent print quality it is desirable that the ink drops jetted onto the blanket spread and become well-coalesced prior to drying. Otherwise, the ink images appear grainy and have deletions. Lack of spreading can also cause failing inkjet ejectors to be much more apparent, producing broader streaks in the ink image. Spreading of aqueous ink is facilitated by materials having a high surface energy.
However, in order to facilitate transfer of the ink image from the blanket to the media substrate after the ink is dried on the intermediate imaging surface, a blanket having a surface with a relatively low surface energy is preferred. Rather than providing the desired spreading of ink, low surface energy materials tend to promote “beading” of individual ink drops on the image receiving surface.
Thus, an optimum blanket for an indirect image transfer process must tackle both the challenges of wet image quality, including desired spreading and coalescing of the wet ink; and the image transfer of the dried ink. The first challenge—wet image quality—prefers a high surface energy blanket that causes the aqueous ink to spread and wet the surface. The second challenge—image transfer—prefers a low surface energy blanket so that the ink, once partially dried, has minimal attraction to the blanket surface and can be transferred to the media substrate.
Various approaches have been investigated to provide a solution that balances the above challenges. These approaches include blanket material selection, ink design and auxiliary fluid methods. With respect to material selection, materials that are known to provide optimum release properties include the classes of silicone, fluorosilicone, a fluoropolymer, such as TEFLON or VITON, and certain hybrid materials. These materials have low surface energy, but provide poor wetting. Alternatively, polyurethane and polyimide have been used to improve wetting, but at the cost of ink release properties. Tuning ink compositions to address these challenges has proven to be very difficult since the primary performance attribute of the ink is the performance in the print head. For instance, if the ink surface tension is too high it may not jet properly, depending on type of printheads, and it if is too low it may drool out of the face plate of the printhead.
In addition to affecting image quality and transfer characteristics of the ink, the sacrificial coating properties can also affect water fastness of the prints. Water fastness is a known concern for aqueous inks generally. Poor water fastness can result in smudging, reduced image quality and unwanted transfer of ink such as to the fingers of users handling the images.
Identifying and developing new polymer coating materials that provide good wet image quality and/or image transfer with improved water fastness would be considered a welcome advance in the art.